1. Field of the Invention
The present invention relates to an exposure apparatus in which variations in imaging characteristics are reduced.
2. Description of the Related Art
A process of manufacturing a semiconductor device such as an LSI or VLSI formed from an extra fine pattern has conventionally adopted a reduction projection exposure apparatus which reduces and prints by exposing a circuit pattern drawn on a mask on a substrate (also called a “wafer”) coated with a photosensitive agent, thereby forming a desired pattern on the substrate. Along with an improvement in the packaging density of semiconductor devices, further micropatterning is becoming necessary. A demand for micropatterning on the exposure apparatus is increasing along with the development of the resist process.
To improve the resolving power of an exposure apparatus, there are a method of shortening the exposure light wavelength and a method of increasing the numerical aperture (NA) of a reduction projection lens. As the resolving power increases, the depth of focus of the reduction projection lens decreases. It is therefore important to improve the focus accuracy of focusing the wafer surface on the imaging plane (focal plane) of the reduction projection lens. One important optical characteristic of the projection exposure apparatus is the alignment accuracy of precisely aligning various patterns obtained by a plurality of processes. An important factor which influences the alignment accuracy is a magnification error of the reduction projection lens. Along with a stronger trend toward further micropatterning of a VLSI every year, a need for improving the alignment accuracy is becoming stronger. It is therefore very important to maintain the magnification of the reduction projection lens at a predetermined value.
A reduction projection lens is known to partially absorb exposure energy so that the temperature of the reduction projection lens changes due to heat generated by the absorption, leading to a change in the optical characteristics of the reduction projection lens, such as the refractive index. When the reduction projection lens is continuously irradiated with exposure light for a long period of time, the imaging characteristics (including at least one of the focus, magnification, and wavefront aberrations such as astigmatism aberration and distortion aberration) of the reduction projection lens vary. This may result in non-negligible amounts of focus errors or alignment errors. Under the circumstance, there is proposed a method of correcting variations in imaging characteristics, which occur when exposure energy is applied to the reduction projection lens.
For example, the applicant of Japanese Patent Publication No. 63-16725 proposes calculating the amounts of variations in imaging characteristics, which occur when exposure energy is applied to the reduction projection lens by model equations using, e.g., the exposure amount, exposure time, and non-exposure time as variables. Based on the calculation result, the variations in imaging characteristics of the projection optical system are corrected. The model equations have imaging characteristic-specific coefficients unique to the reduction projection lens. Measuring the coefficients makes it possible to correct the variations in the imaging characteristics of the reduction projection lens.
There is also proposed an exposure apparatus which can obtain a more excellent projection resolving power for a specific pattern by changing the illumination shape. In such an apparatus, a light source distribution generated on the pupil surface of the reduction projection lens changes depending on the exposure conditions (e.g., the numerical aperture of a projection system, the numerical aperture of an illumination system, the exposure area, the exposure center position, and the exposure mask). Therefore, the variation amounts of imaging characteristics change for the respective exposure conditions.
Under the circumstance, there is proposed an exposure method of satisfactorily adjusting the variations in imaging characteristics even when the distribution of energy applied to the reduction projection lens changes. For example, Japanese Patent No. 2828226 proposes a method of storing imaging characteristic correction coefficients corresponding to various states of the illumination light source distribution, and reading out corresponding correction information when the state of the light source distribution is changed, performing correction based on the readout information. To precisely correct variations in imaging characteristics corresponding to various states of the illumination light source distribution, it is necessary to calculate a correction coefficient best suited to a set of exposure conditions of interest from pieces of information on, e.g., the state of the illumination light source distribution on the pupil plane, the reticle transmittance, the dimensions of the exposure area in the scanning direction and in a direction perpendicular to the scanning direction, the scanning speed, the exposure amount, and the irradiation time.
It is necessary to calculate a correction coefficient best suited to a set of exposure conditions of interest. For this purpose, the transmittance of a mask needs to be calculated from mask transmittance information (e.g., mask transmittance map information/mask design information) and information on the exposure area on the mask. However, the prior arts do not take account of the difference in transmittance (the difference in pattern density) attributed to the image height in the illumination area. Still worse, even when the pattern density in the illumination area is calculated from the mask transmittance information and illumination area information, correction systems in the prior arts have poor correction capabilities.
Nowadays, however, the correction systems are being upgraded to meet a demand for an improvement in the accuracy of the exposure apparatus. This has made it possible to correct variations in imaging characteristics which depend on the image height in the illumination area.